Numerous apparatuses have been described in the literature whose purpose is to contain a boiling liquid fluid and the condensable vapor of the fluid in a vessel open to the atmosphere at the top. Open signifies that no doors or locks are used to contain the vapor. The vessel, also known as a sump, has a source of heat that boils the liquid. At the top of the vessel there is some form of condensing apparatus to condense the vapor. This condensing apparatus is typically in the form of cooling coils or a cooling jacket. Typically, chilled water is used as the cooling medium.
Another form of this type of apparatus has a weir, a projection of a specific length, at the top of the vessel that allows the dense vapors of the fluid to "spill" over the weir into a condensing apparatus. The condensed vapor is normally returned as liquid to the sump for reboiling.
The above described apparatuses presume that the fluid has a vapor density greater than that of the surrounding atmosphere. Consequently, a stable horizontal interface is formed between the condensable fluid vapor beneath the atmosphere (typically ambient air). The vapor-interface is a narrow, stable region if the density of the vapor is significantly greater than the ambient air because the system is stably stratified.
When such an apparatus is used for assembly cleaning, is is known as a vapor degreaser and the condensable fluid is typically a fluorinated or chlorinated solvent or an azeotrope of such a solvent. Examples of such a fluid are dichlorodifluromethane, 1,1,1,-trichloroethane, or an azeotrope of dichlorodifluoromethane and methanol. The vapors of these solvents are much denser than ambient air. In these systems, a boiling sump typically contains the solvent along with contaminants removed in a cleaning process; the vapor and the condensate are highly purified.
The assembly to be cleaned is typically lowered into the vapor space by passing it through the interface between the solvent vapor and ambient air. The assembly is then either immersed in liquid solvent or sprayed with liquid solvent to remove most of the contaminant. It is then withdrawn to the vapor where a small amount of clean vapor is condensed on the assembly to provide a final rinse. The assembly is then withdrawn through the stable interface between the solvent vapor and ambient air. Since the assembly has been heated by the condensing vapor to a temperature approaching the atmospheric boiling temperature of the solvent, the remaining liquid solvent on the assembly normally vaporizes rapidly into the ambient atmosphere following withdrawal from the solvent vapor.
When such an apparatus is used for melting or reflowing solder, it is known as a condensation or vapor phase soldering machine. In vapor phase soldering, the condensable fluid is typically a perfluorinated fluid with a boiling point in excess of the melting point of the solder. The vapors of these fluids are much denser than ambient air. In these systems the assembly to be soldered typically is lowered into the vapor space by passing it through the interface between the fluid vapor and ambient air. The assembly is rapidly heated by the latent heat of vapor condensing on its surface until the assembly's temperature approaches the atmospheric boiling temperature of the fluid which exceeds the melting point of the solder on the assembly. The assembly is then withdrawn into the air.
The concept of stable stratification can be used to introduce a secondary condensable vapor into the system as in the case of a secondary vapor blanket for condensation soldering. In this case, the primary vapor is of greater density than a secondary vapor which is of greater density than the ambient air. The secondary vapor, therefore, retains the primary vapor in the vessel.
During the 1980s it was discovered that many chemically stable chlorinated and fluorinated fluids had the potential of destroying the ozone in the stratosphere. Models have shown that these chemically stable halogenated fluids can migrate to the stratosphere where they act as catalysts for rapidly depleting ozone. These fluids are frequently referred to as ozone depleting chemicals. The Montreal Protocol, the United States Clean Air Act of 1990 and other legislation and regulations throughout the world have established a strict time schedule to eliminate the use of such fluids. In addition, environmental concerns over the impact of volatile organic compounds and their effect of producing smog and creating ozone in the lower atmosphere have resulted in legislation and regulations controlling the emissions of such organic fluids as alcohols in addition to the halogenated fluids. Consequently, there has been significant research and development to find alternative fluids to use in a variety of manufacturing processes including cleaning, coating, and soldering.
One common fluid of interest for cleaning and coating applications is water. Water is a good solvent for ionic contaminants and, when used in conjunction with surfactants, detergents, terpenes, etc., can be used to remove organic contaminants. It would be desirable to use a vapor containment apparatus as described above with liquid water and steam; such a system would contain a sump with boiling water that might contain contaminants or other processing chemicals such as surfactants or detergents and it would generate a vapor space with purified water vapor.
It is difficult, however, to contain the steam vapor in a chamber open to the atmosphere; water boiling in a pot "plumes" into the room because its density is less than ambient air. There is a resulting need for an apparatus that contains vapor having a density less than air and is open to the ambient air, thus allowing easy entrance and exit of assemblies for processing within the apparatus.